1. Guangdong Provincial Center for Disease Control and Prevention, Guangzhou, Guangdong Province, People's Republic of China.
2. Office of Disease Control and Emergency Response, Chinese Center for Disease Control and Prevention, Beijing, People's Republic of China.
3. U.S. Centers for Disease Control and Prevention, Atlanta, Georgia.
4. State Key Laboratory for Infectious Disease Prevention and Control, National Institute for Communicable Disease Prevention and Control, Chinese Center for Disease Control and Prevention, Beijing, People's Republic of China.